1. Field of the Invention
The present invention relates to an internal combustion engine.
2. Description of the Related Art
In the past, in an internal combustion engine, for example, a diesel engine, the production of NO.sub.x has been suppressed by connecting the engine exhaust passage and the engine intake passage by an exhaust gas recirculation (EGR) passage so as to cause the exhaust gas, that is, the EGR gas, to recirculate in the engine intake passage through the EGR passage. In this case, the EGR gas has a relatively high specific heat and therefore can absorb a large amount of heat, so the larger the amount of EGR gas, that is, the higher the EGR rate (amount of EGR gas/(amount of EGR gas+amount of intake air), the lower the combustion temperature in the engine intake passage. When the combustion temperature falls, the amount of NO.sub.x produced falls and therefore the higher the EGR rate, the lower the amount of NO.sub.x produced.
In this way, in the past, it was known that the higher the EGR rate, the lower the amount of NO.sub.x produced can become. If the EGR rate is increased, however, the amount of soot produced, that is, the smoke starts to sharply rise when the EGR rate passes a certain limit. In this point, in the past, it was believed that if the EGR rate was increased, the smoke would increase without limit. Therefore, it was believed that the EGR rate at which smoke starts to rise sharply was the maximum allowable limit of the EGR rate.
Therefore, in the past, the EGR rate was set within a range not exceeding the maximum allowable limit. The maximum allowable limit of the EGR rate differed considerably according to the type of the engine and the fuel, but was from 30 percent to 50 percent or so. Accordingly, in conventional diesel engines, the EGR rate was suppressed to 30 percent to 50 percent at a maximum.
Since it was believed in the past that there was a maximum allowable limit to the EGR rate, in the past the EGR rate had been set within a range not exceeding that maximum allowable limit so that the amount of NO.sub.x and smoke produced would become as small as possible. Even if the EGR rate is set in this way so that the amount of NO.sub.x and smoke produced becomes as small as possible, however, there are limits to the reduction of the amount of production of NO.sub.x and smoke. In practice, therefore, a considerable amount of NO and smoke continues being produced.
In the process of studying the combustion in diesel engines, however, it was discovered that if the EGR rate is made larger than the maximum allowable limit, the smoke sharply increases as explained above, but there is a peak to the amount of the smoke produced and once this peak is passed, if the EGR rate is made further larger, the smoke starts to sharply decrease and that if the EGR rate is made at least 70 percent during engine idling or if the EGR gas is force cooled and the EGR rate is made at least 55 percent or so, the smoke will almost completely disappear, that is, almost no soot will be produced. Further, it was found that the amount of NO.sub.x produced at this time was extremely small. Further studies were engaged in later based on this discovery to determine the reasons why soot was not produced and as a result a new system of combustion able to simultaneously reduce the soot and NO.sub.x more than ever before was constructed. This new system of combustion will be explained in detail later, but briefly it is based on the idea of stopping the growth of hydrocarbons into soot at an intermediate stage before the hydrocarbons grow.
That is, what was found from repeated experiments and research was that the growth of hydrocarbons stops at an intermediate stage before becoming soot when the temperature of the fuel and the gas around the fuel at the time of combustion in the combustion chamber is lower than a certain temperature and the hydrocarbons grow to soot all at once when the temperature of the fuel and the gas around the fuel becomes higher than a certain temperature. In this case, the temperature of the fuel and the gas around the fuel is greatly affected by the heat absorbing action of the gas around the fuel at the time of combustion of the fuel. By adjusting the amount of heat absorbed by the gas around the fuel in accordance with the amount of heat generated at the time of combustion of the fuel, it is possible to control the temperature of the fuel and the gas around the fuel.
Therefore, if the temperature of the fuel and the gas around the fuel at the time of combustion in the combustion chamber is suppressed to no more than a temperature at which the growth of the hydrocarbons stops midway, soot is no longer produced. The temperature of the fuel and the gas around the fuel at the time of combustion in the combustion chamber can be suppressed to no more than a temperature at which the growth of the hydrocarbons stops midway by adjusting the amount of heat absorbed by the gas around the fuel. On the other hand, the hydrocarbons stopped in growth midway before becoming soot can be easily removed by after-treatment using an oxidation catalyst etc. This is the basic thinking behind this new system of combustion.
In an internal combustion engine using this new system of combustion, however, not only oxidation catalysts, but various other types of after-treatment devices are used to clean the exhaust gas. In this case, among these after-treatment devices, there are after-treatment devices which require that the temperature of the after-treatment device be maintained at a high temperature over a long period of time. Therefore, when using such an after-treatment device, it is necessary to maintain the temperature of the after-treatment device at a high temperature for a long time.
On the other hand, as explained later, if this new combustion is performed, the temperature of the exhaust gas becomes higher than with the conventionally performed combustion and therefore if this new combustion is used, it becomes possible to maintain the after-treatment device at a high temperature for a long period. This new combustion, however, can only be used when the engine load is relatively low. When the engine load becomes high, the conventionally performed combustion has to be used. When using the conventionally performed combustion, however, the temperature of the exhaust gas is low and therefore there is the problem that when switching from the new combustion to the conventionally performed combustion, the after-treatment device cannot be maintained at a high temperature for a long period since the temperature of the exhaust gas falls.